Switching regulators are voltage regulators incorporated by many switch-mode power supply (SMPS) applications. Typically, a switching regulator uses a power switch, an inductor, and a diode to transfer energy from input to output. A pulse-width-modulation (PWM) digital control signal is used in the switching regulator to generate a regulated output voltage. Among different types of switching regulators, a flyback converter is a versatile regulator with an inductor split to form a transformer which provides an additional advantage of input to output isolation.
FIG. 1 (prior art) illustrates an exemplary conventional primary-side regulation (PSR) flyback converter 10. Converter 10 includes a transformer 11, an external NPN bipolar transistor 15 and a controller integrated circuit (IC) 16. Transformer 11 has three windings: a primary-side winding 12, a secondary-side winding 13, and an auxiliary winding 14. Controller IC 16 includes a pre-amplifier 21, a sampler 22, a pulse-width-modulation (PWM) error amplifier 23, an error comparator 24, an oscillator & tramp detector 25, PWM control logic 26, a gate driver 27, a current-sense amplifier 28, a MOSFET 29, an internal main power switch 30, and a current-sense resistor 31.
When main power switch 30 is turned on, an input voltage VIN is forced across the primary inductor 12 and an inductor current 17 ramps up through primary inductor 12. A magnetic field is generated that transfers energy to secondary winding 13 when main power switch 30 is turned off. The energy transferred to the secondary winding 13 is output from converter 10 as a secondary inductor current ISEC with an output voltage VOUT. The voltage VAUX on auxiliary winding 14, an indication of the output voltage VOUT, is fed back via auxiliary winding 14 and feedback terminal FB 18 to controller IC 16. Inside controller IC 16, auxiliary feedback signal 41 (VFB) is amplified against a reference voltage VREF by pre-amplifier 21 to produce a feedback error signal 42 (VERROR). VERROR is sampled by sampler 22 and is further amplified by PWM error amplifier 23. Error comparator 24 receives error amplifier output signal 43 and current-sense signal 44 and outputs a regulation signal 45. Regulation signal 45 is used to set the on-time of main power switch 30 through PWM control logic 26. The output voltage VOUT is thus regulated through such feedback and control mechanism.
Because of the feedback and control mechanism, PSR flyback converter 10 relies on feedback error signal VERROR to regulate the output voltage VOUT. Feedback error signal VERROR, however, is sensitive to the ISEC·RSEC voltage drop resulting from the secondary inductor current loop total parasitic resistance RSEC. The output voltage VOUT therefore cannot be accurately regulated without compensating for such ISEC·RSEC voltage drop. This is especially true when PSR flyback converter 10 operates in a continuous conduction mode (CCM) because the secondary inductor current ISEC varies widely in CCM operation. A method is sought for compensating the secondary inductor current loop resistance RSEC in order to more accurately regulate the output voltage VOUT of a switching regulator that operates in CCM.